Propeller pitch change mechanism



May 28, 1957 I J. M. MERGEN ET AL 2,793,703

PROPELLER PITCH CHANGE MECHANISM Original Filed Feb. 11, 1950 4 Sheets-Sheet 1 I 7' JOJ/SWM M59601 3mm HOWA/Pfl MURPHY 3% WWW y 1957 J. M. MERGEN El AL 2,793,703

PROPELLER PITCH CHANGE MECHANISM Original. Filed Feb. 11, 1950 4 Shee ts-Sheet 2 May 28, 1957 J. M. MERGEN ET AL 2,793,703

PROPELLER PITCH CHANGE MECHANISM Original Filed Feb. 11, 1950 4 Sheets-Sheet 5 a M R WM JOSEPH M MEPGf/V HOWARD MUPP/HV May 28, 1957 J. M. MERGEN ET AL 2,793,703

PROPELLER PITCH CHANGE MECHANISM Original Filed Feb. 11, 1950 4 Sheets-Sheet 4 V; W m? w A 0 MR MW W m w SW O 2,793,703 PROPELLER PITCH CHANGE MECHANISM Original application February 11, 1950, Serial No. 143,636. Divided and this application June 23, 1953, Serial No. 363,584

1 Claim. (Cl. 170--160.29)

This invention relates to pitch changing mechanisms for controllable pitch aeronautical propellers. In particular, the invention relates to pitch changing mechanisms of the type in which powerfor pitch change is derived principally from the power plant driving the propeller, pitch-changing power being obtained by the clutching of the driving member to gearing connected with the propeller blades. Clutch mechanisms are provided for in creasing pitch and for decreasing pitch; a brake is provided to hold fixed pitch; and control mechanism is provided' to energize the clutches electrically, intermittently if desired, in accordance with demands for pitch change at ditferent rates. This application is a division of our application Serial No. 143,636 filed February 11, 1950, now Patent No. 2,738,045.

This invention in a general sense, has features in common with the propeller pitch change mechanism as shown and described in application Serial No. 73,586 filed Ianuary 29, 1949, now Patent No. 2,646,131, assigned to Curtiss-Wri'ght Corporation, insofar as there are similarities in the intermittent pitch control mechanisms in this and the prior inventions. However, in said pr'ior application the pitch change mechanism and clutches were operated hydraulically while in the present invention, they are operated electrically.

The present invention includes novel types of electrically actuated clutches and brakes to connect and disconnect the prime mover relative to the propeller pitch change mechanism. These clutches comprise servo mech anisms actuated by eddy current devices whereby relatively small amounts of electrical energy may be utilized for control purposes to actuate clutches having high torque capacity.

Objects of the invention include the provision of means in a controllable pitch propeller for obtaining great flexibility of operation, for attaining variable rates of pitch change of the propeller, for providing selection of dilferent modes of operation of the propeller, and for providing the advantages of electrical pitch control with itsinher ent lightness and freedom from service difficulties.

While there are in the prior art, propellers which utilize prime mover power for effecting pitch change through clutching arrangements, and propellers which incorporate various safety provisions and means for attaining several desired regimes of operation, the propeller system in this invention is believed to attain these several operational advantages in a distinctly improved fashion, by virtue of the electrical control system, the electrically operated clutches, and other mechanisms present in the system.

The nature of the invention may be appreciated in detail by reading the annexed detailed description in connection with the drawings in which Fig. l is a longitudinal section through one of the eddy current operated clutch units' of the invention;

Fig. 2 is a longitudinal section through an alternative form of eddy current operated clutch unit which incorporates a coincidentally operable brake;

Patented May 28, 1957 Fig. 3 is a schematic diagram of a propeller system, incorporating the clutch unit of Fig. 1;

Fig. 4 is a schematic diagram of an alternative arrangement of the invention incorporating the clutch and brake units of Fig. 2;

Fig. 5 is a perspective elevation of part of the pitch change drive gearing as utilized in the system of Fig. 4 incorporating the clutch and brake units of Fig. 2;

Fig. 6 is a typical section on the line 66 of Fig. 2;

Fig. 7 is a section similar to Fig. 6 showing the mechanism adjusted to a different position; and

Fig. 8 is a typical section on the line 8-8 of Fig. 2 showing the arrangement of one form of a centering or disengaging spring for a clutch unit.

Reference may first be made to Figs. 1 and 3 with particular emphasis on Fig. 3. Therein, a propeller hub 10 incorporates a plurality of blade sockets, one of which is shown at 12, a blade 14 being suitably mounted in each socket. At its inner end the blade is provided with a worm wheel 16 engaged by a worth 18 driven by a shaft 20 engaging a gear 22 concentric with the hub and carried on a sleeve 24 rotatable with or with respect to the hub. The inboard end of the sleeve carries a gear 26 forming part of a transfer gearing including a fixed internal gear 30 coplanar with the gear 26. Planet pinions 32 are engaged between the gears 26 and 30, these planet pinions being respectively coaxial and rotatable with planet pinions 34. The latter pinions are embraced by an internal control gear 36, the pinions engaging a sun gear 38 keyed to or driven by the propeller shaft 40. This transfer gearing 28 is somewhat similar to others which have appeared in the prior art and operates as follows: If the control gear 36 is fixed, the sun gear 38 will drive the planet pinions 32, 34 at some speed less than propeller speed and the sun gear 26 will be enforced to rotate at propeller speed because the internal gear 30 is anchored. Thus, the sleeve 26 will rotatewith the propeller and no pitch change will be imparted to the propeller blades. If the control gear 36 is rotated in either direction, the sun gear 26 will necessarily be rotated in one direction or the other with respect to the propeller shaft and pitch change will be imparted to the propeller blade.

To eifect rotation of the blade pitch control gear 36, a gear 42 is provided on the propeller shaft which is meshed with a gear 44 of a clutch unit 46. The clutch is adapted to be engaged by means to be described to enforce increased pitch operation of the propeller. The clutch when engaged drives a pinion 48 meshed with external teeth on the pitch control gear 36 to increase propeller pitch. The propeller shaft gear 42 also drives an idler 50 whichin turn drives the gear 57. on a decrease pitch clutch 54, thelatter having an output: pinion 56 engaged withthe external teeth on the pitch control. gear 36. When the clutch 54 is engaged, decrease pitch movement is enforced on the gear36. When both clutches 46 and 54 are disengaged, no pitch change is enforced and during this period, the gear 36 must be braked so that pitch changes will not occur from aerodynamic efiects on the propeller blades or other causes. To this end, an electrically actuated brake 58 is provided, including a gear 60 engaged with the pitch control gear 36. This brake,which need not be described in detail, includes a solenoid actuated disc brake assembly, one of the disc sets carrying the pinion 60 and the other set being anchored. against rota- 4 tion. The brake is normally spring pressed into. braking engagement and the solenoid when energized, overcomes the springs and releases the brake. clutches and brake on a coincidental basis is provided by the control mechanism to be described. When propeller rotational speed is low, or where the'j propeller is not rotating, pitch change is: not effected, through the clutches and the gearing above described. A"

Operation of the f 3 small relatively low-powered reversible electric motor 62 is provided, having an output gear 64 meshing with the pitch control gear 36. This motor is utilized principally for. the final stages of. feathering and the initial stages of unfeathering and for changing propeller pitch when the propeller is not being rotated.

Some. of the important features of the invention reside in the clutch units 46 and 54, shown in detail in Fig. l, and these units will now be described. The units 46 and 54 are. substantially identical so that reference characters used for the unit 46 are also applicable to the clutch unit at 54. The unit consists of a fixed pilot shaft 68 secured to a. housing 70 through. the medium of an electro-magnetic unit 72, this number also being utilized to designate the iron core of the unit. A plurality of alternate overlapping, pole pieces 74 are arranged around the core 72 and likewise embrace a winding 76 which may be electrically energized. When the winding is energized, high density magnetic flux. is built up in the elements 72 and 74 establishing. a plurality of peripheral north and south poles. Embracing the elements 72 and 74 is a free running cylinder 78 having. clearance relation to the periphery of the pole pieces 74. When the coil 76 is deenergized, the cylinder 78 may rotate freely, with no drag, around the pole pieces but when the coil 76 is electrically energized, eddy currents are set up in the cylinder 78 creating a torque between the pole pieces 74 and the cylinder 78 tending to stop rotation of the cylinder. The amount of torque attainable depends upon the size of the elements, their relative speed, and the flux density established by the electrical energization and these characteristics are so chosen that the retarding torque imposed on the cylinder 78 is sufiicient to energize a servo mechanism now' to be described.

The cylinder 78 is carried on a diaphragm 80, antifrictionally borne as at 82 on a sleeve 84. The sleeve 84, relatively rotatable to the cylinder 78, is provided with spokes 86 and a rim 88 on which the gear 44 is formed. This rim incorporates internal splines 90 to which are fitted clutch plates 92. To the diaphragm is. secured a member 94 through projections which pass between the spokes 86. On the leftward surface of the member 94, as shown, is formed a waved substantially annular cam track 95. Opposite this track is a similar track 96 on a member 98 whose periphery engages the splines of the rimv 88. Balls 100 are disposed between the members 98 and 94, bearing on the cam tracks, and are retained by a perforated retaining ring 102 which is position-controlled by a ditferential unit 104 comprising pinions on the retainer engaging. face gears on the elements 94 and 98 to afford proper position control of the balls 100 between the cam tracks. The balls are so located, referring briefly to Fig. 6, that they will normally be located in the bottoms of the recesses of both cam tracks, to allow axial positioning of the member 96 to the right, as shown in Fig. 1, when the clutch is not energized.- This relieves engaging pressure between the clutch plates 92 and 106, the latter being splined at 108 to the driving member 110 carrying the clutch output gear 48. When the coil 76 is energized, a dragging or retarding force is imposed on the sleeve 78 enforcing an angular displacement between the members 94 and 98 as shown in Fig. 7. Thereupon, the balls 100 roll along the cam tracks 95 and 96 to enforce leftward movement of the member 98, pressing the clutch plates 92 and 106 toward one another to enforce clutch engagement and to lock the input gear 44 and output gear 48 for unitary rotation with one another.

Upon relaxation of energization of the coil 76, the sleeve 78 is free to rotate with the clutch input gear 44. To afford clutch disengagement, a plurality of annular waved springs 112 are disposed. between the clutch plates 92. These springs urge these plates apart and are of sufiicient. power to rotate the members 94 and 98 with.

respect. to each other, when the. coil 76 is deenergized, to bring the balls 100 and the cam tracks of the members 4 94 and 98 into the position of Fig. 6, disengaging the clutch plates 92 and 106.

The force imposed by the springs 112 is carefully selected to overcome the friction in the system involving the elements 94, 96, 100, 104, 82 and 78 to disengage the clutch, but is small enough to be overcome when the eddy current clutch is energized.

The configuration and design of the eddy current clutch unit shown in Fig. 1 is susceptible of considerable modification. As shown and as utilized in the propeller system of Fig. 3, both the driving and driven members of the clutch unit are normally rotated. If the environment calls for different arrangements, the system can be organized as a brake wherein one member is fixed and the other rotates, and various devices may clearly be made from the design shown. It is pointed out however, that under normal operating conditions the eddy current sleeve 78 and the pole pieces 74 should normally rotate with respect to one another so that a control torque may be developed therebetween on energization of the coil 76. The eddy current device inherently allows slip between the relatively rotating members even when energized and does not in itself provide a positive clutching between the members. It is this characteristic which makes the eddy current clutch controller particularly desirable in the environment herein described, for small amounts of electrical power maybe utilized to maintain control effect upon a high capacity clutch, the clutch itself being capable of positive unitary lock-up of a driving member relative to a driven member. If the eddy current device were to be used by itself to afiord clutching between the driving and the driven member and to develop suflicient torque to afford the necessary pitch control, the eddy current device would have to be unduly magnified in weight and size to the point where it could not be utilized satisfactorily in an aircraft propeller. Doubtless it is true that there are many other environments besides aircraft propellers wherein size and weight limitations of eddy current clutches would be prohibitive unless they are used as energizing means for' powerful positive clutches.

Reference is again made to Fig. 3 which shows the electrical control arrangements for the several components heretofore described including the clutches 46 and 54, and brake 58, and the auxiliary motor 62. A control box of any appropriate type is shown at 116 having an output member 118 which calls for no pitch change, increased pitch, or decreased pitch, the pitch change called for being variable in rate. The control box 116 may comprisea governor or a regulator system, not a part of this invention, adapted for propeller control derived from engine R. P. M. or other qualities such as those which are characteristic either of reciprocating engines or of internal combustion turbines. As shown, the control member 118 calls for increased pitch at a rate somewhere near the maximum rate attainable. The functioning of the system in. response to this call along with a description of the elementsof the system will now be undertaken.

The control elements 118 engages a groove 120 formed on a control shaft 122, said shaft being driven from a propeller shaft gear 124 which engages a gear 126 coaxial and keyed to the shaft 122 and with respect to which the shaft 122 is axially movable. As the system is shown in Fig. 3, the propeller is operating at a low speed or is stationary. The shaft 122 carries a non-conducting sleeve 128 and bordering conducting sleeve 130 and 132, the conducting sleeves being engaged by switch leaves 134 and when the control shaft 122 is centrally positioned and the leaves 136 and 138 engaging the con ducting sleeves. When the control shaft 122 is moved for pitch change, both leaves 138 and 140 will bear on the sleeve 132 or both leaves 134 and 136 will bear on the sleeve 130. The leaves 134 and 138 arc connected respectively to the coils 76 of the increase pitch and decrease pitch clutches 46 and 54 and are also connected at 142 and 144 to the increase and decrease pitch windings respectively of the auxiliary motor 62.

The common lead 146 of the auxiliary motor is connected to a switch point 150 on a centrifugally operated switch 152 having a contact 154 connected to a master switch 148 connecting to a battery or other power source 160, the other end of the battery being grounded. The speed responsive switch 152 includes a movable arm 162 operated by a flyweight assembly 164 driven from the propeller shaft 40 by gears 166. This assembly is so arranged that at speeds below about 200 R. P. M. the points 150 and 154 of the switch 152 are connected together by the element 162. At speeds above about 200 R. P. M. the element 162 rises and connects switch point 168 and 170 together. In the position shown, the RP. M. is presumed to be less than 200 R. P. M. so that the battery 160 is connected through the centrifugal switch to the auxiliary motor 62, the pitch controller 116 calling for increased pitch. The pitch increasing winding of the auxiliary motor 62 is connected through the leaves 134 and 136, through a high pitch limiting switch 170, through the brake 58 and thence to ground. Thus, the auxiliary motor 62 is energized to increase pitch and brake 58 is released to allow of pitch change. Increasing pitch change will occur so long as there is a call therefor from the control unit 116, and until the high pitch limit is reached whereupon the limit switch 170 will open and the electrical circuit becomes inactive. Thereafter, should a decrease pitch call be made by the controller 116, the decrease pitch Winding of the auxiliary motor 62 will be energized through the lead 144 and the leaves 138 and 140, concurrently with energization of the electromagnetic brake S8 effecting a pitch reduction in the propeller.

When the propeller is rotating at more than 200 R. P. M. the speed responsive switch 152 is adjusted to interconnect the points 170 and 168, thereby cutting out potential operation of the auxiliary motor 62. This brings into action a cam switching unit 1'74 operable, in response to control demands of the control unit 116, to engage either the pitch increase and pitch decrease clutches 46 and 54, coincidentally with disengagement of the electromagnetic brake 58. It will be seen that power from the 'battery 160 is available to the switching unit 174 when the speed responsive switch 152 interconnects the points'170 and 168. The unit 174 consists of axially spaced cams 176 and 178 on the engine driven shaft 122, these cams having facing slopes 180, the slopes terminating in steps 182 parallel to the axis of the shaft 122. A decrease pitch cam follower 184 and an increase pitch cam follo-wer 186 are pivotably mounted on a rod 188, these followers being spaced apart a sufficient distance so that, when the pitch controller 116 calls for no pitch change, both followers ride on the cylindrical portions of the cams176 and 178. In this condition, as the shaft 122 rotates, the cam followers 184 and 186 ride the surfaces of the cylinders of the cams and do not oscillate.

' A switch 190 is operated by the follower 184 and a switch 192 is operated by the follower 186, .both of these switches being open in the above described operating condition calling for no pitch change. Should there be a demand for increased pitch, the shaft 122 is moved axially to the right as shown and as the shaft rotates, the follower 186 periodically drops off the cam slope 180 and is raised by the cam step 182. This oscillation of the follower 186 periodically closes the switch 192, the dwellof closure depending upon the axial displacement of the shaft 122. If the axial displacement is extreme, farther to the right than the position shown in the figure, the 'cam face 180 will drop the cam follower 186 for relatively long periods allowing the switch 192 to remain closed for a long period. In similar fashion, if decreased pitch is called for, the cam follower 184 is permitted to oscillate and to close the switch 190 cyclically, the closure dwell depending up n the rate of decreased pitch called for bythe controller 116.

Upon closure of the switch 192, the pitch increasing clutch 46 is energized, causing power to be transmitted from the propeller shaft to the pitch control gear36. The return circuit of the clutch 46 passes through the increase pitch limit switch 170 to the electro-magnetic brake 56 and thence to ground so that the clutch 46 and the brake 58 are energized simultaneously engaging the clutch and disengaging the brake. If the propeller pitch reaches the maximum limit, the limit switch 170 is opened so that no further pitch changing action takes place.

When decreased pitch change is called for by closure of the switch 190, the decrease pitch clutch is energized simultaneously with energization of the brake 58, through the limit switch 196, causing pitch decrease.

A call for feathering of the propeller may be made by the controller 116. Initial feathering is accomplished by the clutch 46 until the propeller R. P. M. decreases to the point where the speed responsive switch. 152 transfers pitch change actuation from the clutch units to the auxiliaiy motor 62 which then accomplishes final feathering. A call for unfeathering is made by the controller 116 which energizes the auxiliary motor 62 to initiate unfeathering. As pitch of the propeller blades decrease, the power plant is rotated by wind-milling action on the propeller, gradually incneasing the R. P. M. thereof until the speed responsive switch 152 restores pitch. control to the clutches 46 and 54, completion of the unfeathering operation being obtained from the decrease pitch clutch 54.

If it is desired to operate the propeller in fixed pitch, the master switch 148 is opened which disconnects the battery from the speed responsive switch 152, preventing disengagement of the brake: 58, and preventing operation of the motor 62 at low engine speed or of the clutches 46 and 54 at high engine speed.

By appropriate operation of the pitch controller 116, reverse pitch positions of the propeller 10 may be attained merely by calling for decrease pitch operation until reverse pitch conditions are reached. Pitch adjustment may be attained in the reverse pitch quadrant of propeller blade adjustment by appropriate manipulation of the controller 116. Ordinarily, reverse pitch operation is used only with the power plant in operation so that the reverse pitch adjustments are obtained by application of engine power through the clutches 46 and 54. However, if the power plant were stopped while the propeller pitch adjustment is in the reverse quadrant, normal increase pitch settings are obtainable by calling for a pitch increase wheeupon the auxiliary motor 62 changes propeller pitc Reference may now be made to Figs. 2 and 4-8 in which an alternative embodiment of the invention is disclosed. The principle difference between this embodimerit and the one previously described is in the substitution of two clutch-brake units 200 and 202 (Fig. 4) for the clutch units 46 and 54 and the brake unit 58 of Fig. 3. The construction of the combined clutch-brake units 200 and 202, which are similar to one another, is shown in Fig. 2. The unit 200, namely the increase pitch clutchbrake unit, is selected for detailed illustration and some of the components thereof are found in the schematic arrangement of Fig. 4. The unit includes an input gear 284 driven by the propeller shaft gear 42 and further includes an output gear 206 engaged with the pitch change control gear 36. The unit further includes a gear 208 engaged with the drive pinion 210 of an auxiliary electric motor 212 having increase and decrease pitch field windings and having a coincidentally operable electromagnetic brake therein which engages to lock the drive pinion 210 against rotation when the motor is not energized. Motors of this type are Wellknown in the art and need no further description. When the motor 212 is not energized, its drive pinion 210 forms a reaction gear, locking the gears 208 and 208 of the clutch-brake units 200 and 202 against rotation.

Returning to Fig. 2, the drive pinion 204 is mounted upona sleeve 214 provided with splines 216 carrying clutch plates 218, these p'lates being interleaved with clutch plates 220 splined at their peripheries within a shell 222 carrying the output gear 206. The shell 222 is journalled for rotation concentrically with the shaft 214. The clutch, including the plates 218 and 220, includes an axially movable reaction plate 224 and an axially movable pressure plate 226. The latter plate includes an annular waved cam track 228 engaged by balls 230 which also are engaged by an annular waved cam track 232 formed in a diaphragm 234, said diaphragm carrying an eddy current shell 236. The shell 236 is normally rotated with the drive shaft 214 and is yieldingly held in certain angular relation thereto by a centering spring 238 also shown in Fig. 8, said spring having turnedover ends engaged in slots formed in the member 234, in the member 226, and in a retainer 240 provided for the balls 230. If "a drag force is exerted on the shell 236 tending to retard its rotation behind that of the clutch pressure plate, the spring 238 tends to restore it to centered position but if the drag force is sufficient, the diaphragm will be displaced angularly relative to the pressure plate whereby the balls 230 will force the pressure plate leftwardly as shown to exert clutch engaging pressure thereby connecting the drive shaft 214 to the itput pinion 206.

As noted previously, the reaction plate 224 of the clutch is axially movable so that when the clutch is engaged the reaction plate 224 is pressed leftwardly as shown enforcing leftward movement of pins 242 spaced around and slidable freely in the shell 222. The leftward ends of the pins 242 engage a brake pressure plate 244 which is normally pressed rightwardly by springs 246. The brake pressure plate 244 engages brake plates 248 and 250 respectively splined to a portion of the shell 222 and to a sleeve 252 piloted on the drive shaft 214 through bearings 254. it will be seen that the brake comprising the plates 248 and 250 is normally engaged by means of the springs 246 and is only released when the pressure plate 244 is pressed leftw-ardly as a result of engagement of the clutch plates 220 and 218.

The sleeve 252 normally provides an anchor or reaction member for the brake so that when the brake is engaged, the gear 206 is locked against rotation, preventing any pitch change in the propeller blades. However, there are certain conditions of operation of the propeller when the sleeve 252 is rotatable. To the sleeve 252 is secured the inner race 254 of a one-way roller clutch comprising rollers 256 and an outer cammed race 258, this eammed race 25% carrying exterior gear teeth 208 engaged with the drive pinion 210 of the auxiliary electric motor 212. The direction of action of the one-way clutch is such. as to permit of free rotation of the entire unit shown in Fig. 2 in one direction, even when the brake 248, 250 is locked up.

The decrease pitch clutch-brake units 202 and 200 are exactly the same, as heretofore noted, except that the oneway clutch forming a part of the left hand end gear 208 of the unit ZtlZ is of opposite hand from the one-way clutch in the gear 2% of the clutch-brake unit 200. When the increase. pitch clutch 200 is engaged for increase pitch, clutch unit 202 will be driven in opposite direction from the propeller shaft power gear 42 through the idler 5t and, though the brake thereof is locked up, the entire decrease pitch clutch-brake unit may turn freely through the action of the roller clutch 258'. Conversely, if the decrease pitch clutch-brake unit is energized to effect pitch decrease of the propeller, the unit 200 will spin in its entirety, the one-way clutch 258 overr'unning while the gear 208' is locked from rotation. When neither clutch is energized, both brakes in the units 200 and 202 are locked and no rotation of either can occur since the one-way clutches 258 and 253 oppose one another to prevent rotation.

Ifv according to the control system previously described the auxiliary motor 212 is in operation to change pitch, it

8 will turn one of the units 200 or 202 through the one-way clutch thereof and drive the output gears 206 or 206 through the locked brake of the unit; during this procedure, the unit 202 or 200 respectively will not be effective as a driver since its clutch 258 or 258 respectively will overrun.

In the application of the combined clutch-brake units 200 and 202 in the control system, the arrangements are substantially the same as those previously described with a few minor changes in wiring as will now be pointed out. With the system operating at more than 200 R. P. M. or thereabouts, whereby the switch unit 174 is effective, the intermittent switch 190 is connected directly to the unit 202 and thence through the limit switch 1% to ground. The switch 192 is connected directly to the unit 200 and thence through the other limit switch 170 to ground. Since a brake is incorporated in each of the units 200 and 202, there is no need of carrying the outlet connection from the clutch unit through a separate brake unit to ground. Operation of each unit 200 and 202 is accomplished independently through the existence of the overrunning clutches of the unit not being used.

When the system is operating at less than 200 R. P. M. or thereabouts, pitch control is transferred to the motor 212 whose increase or decrease energization is controlled by the switch leaves 134--140. Return circuits are established through the limit switches 170 and 196, there being no need to series-connect a brake in the line, since the gearing arrangements for the clutches and motor allow the motor to turn the pitch control gear 36 directl Til e two arrangements of the invention described above provide operative embodiments thereof which may be appropriate for certain propeller requirements. Certain features described are amplified, qualitatively, below.

It will be noted that the brake for holding fixed pitch is series-connected either with the clutch units which are engageable to change pitch or with the auxiliary electric motor which may be utilized to change pitch when rotational speed is insufficient to give prompt pitch change from use of the clutches. This provides an outstanding safety feature in a very simple manner, assuring fixed pitch of the propeller should any part of the electrical system, the control system, or the clutches, fail. The brake may only be disengaged upon operation of the clutches or of the auxiliary motor.

By suitable design of the clutch units incorporated in the invention, extremely fast action thereof with little slippage during engagement and disengagement may be attained, so that heat rejection from the clutches will be minimized. It should be clear that the clutches are not intended tooperate on a slipping basis when pitch change is required-rather, they are energized and deenergized to engage and disengage with great speed, affording positive drive when they are engaged.

A further advantage of the system shown is that all electrical apparatus is stationary and connections thereto may be made without the use of slip rings or the like. This feature minimizes service difficulties and further assures consistent operation of the propeller system under high altitude operating conditions, whereat slip ring and brush operation at times becomes erratic and causes obnoxious radio interference. With the stationary arrangement of electrical components, radio interferonce is minimized; the several switches and the like incorporated in the system may be adequately shielded.

While Figs. 3 and 4 disclose a cycling system as part of the propeller pitch changing mechanism, such a cycling system can be eliminated if the propeller control unit such as 116 is such as to produce the desired character of pitch change. For instance, the control box 116 can be directly connected to increase and decrease switch. assemblies to give solid contact for pitch changing functions instead of the intermittent contacts disclosed. A1-

ternatively, the control unit 116, itself, may provide cycling impulses so that a separate cycling machanism becomes unnecessary. The reconnection of the electrical components of the propeller proper, when an alternative type of propeller control is utilized, are comparatively simple and should be obvious to one skilled in the art.

While Figs. 1 and 2 disclose servo clutches actuated by eddy current devices, the invention is intended to comprehend servo clutches which may be operated by other electrically energized drag producing mechanisms such as secondary frictional clutches of low capacity.

Though several embodiments illustrating the invention have been illustrated and described, it is to be understood that the invention may be applied in various forms. Changes may be made in the arrangements shown without departing from the spirit or scope of the invention as will be apparent to those skilled in the art and reference should be made to the appended claim for a definition of the limits of the invention.

What is claimed is:

in a propeller comprising a pitch changing mechanism and blades, said pitch changing mechanism including an input member driven by the propeller and an output member drivably connected to said blades, a normally engaged spring-engaged friction brake engaging said output member and holding it from rotating, electric solenoid means energizable to release said brake, two normally disengaged friction clutches driven in opposite directions by said input member and drivably connected when engaged to said output member, electrically energizable coil means in each said clutch, which, when energized, engages the corresponding clutch to establish a driving connnection from said input member to said output member, an electrical energy source, a circuit closeable to connect one said clutch coil means, said brake solenoid and said energy source in series, another circuit closeable to connect the other clutch coil means, said brake solenoid and said energy source in series and switch means operable to close one or the other of said circuits.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Curtiss Turbo-Electric Propellers, pages 10 and 11, published 1950, by Curtiss-Wright Corp. Propeller Div. (Copy in Div. 22 -16028.) 

